ADP-ribosylation factor domain protein 1 (ARD1) initially cloned in the laboratory, differs from other ARFs by the presence of a 46-kDa amino-terminal extension (p5), which acts as a GTPase-activating protein (GAP) for its ARF domain (p3). Similar to ARF GAPs, the GAP domain of ARD1 contains a zinc-finger motif and arginine residues that are critical for activity. It differs from other ARF GAPs in its covalent association with the GTP-binding domain and the specificity of its GAP activity for the ARF domain of ARD1. ARFs are presumed to play a key role in the formation of intracellular transport vesicles and in their movement from one compartment to another. Both overexpressed and endogenous ARD1 were associated with Golgi and lysosomal membranes, consistent with a role in the formation or function of lysosomes and in protein trafficking between Golgi and lysosomes. Interaction of ARD1 and cytohesin-1 was found in a yeast two-hybrid screen. Cytohesin-2 failed to interact in this system and had much less GEP activity toward ARD1 than did cytohesin-1, although they were equally active with ARF1. Preferential physical interaction was also shown in vitro and residues responsible for specificity of the cytohesin-1/ARD1 interaction were identified. Phenotypic characterization of ARD1 "knock-out" mice began this year. New clues to the physiological function of this protein are emerging.